Active and semi-active vehicular suspension systems have received considerable attention in the past several years as an improvement and replacement to traditional passive systems (Miller and Nobles, 1988; Ivers and Miller, 1989; Jolly and Miller, 1989; Aoyama et al., 1990; Kiriczi and Kashani, 1990; Miller and Nobles, 1990; Kojima et al., 1991; Dunwoody, 1991; Crolla and Abdel-Hady, 1991; Inagaki et al., 1992; Nagiri et al., 1992; Esmailzadeh and Bateni, 1992; Queslati and Sankar, 1992; Pinkos et al., 1993; Hoogterp et al., 1993; Temple and Hoogterp, 1992). Traditional passive systems generally are composed of springs, dampers (shock absorbers), and various structural linkages, such as wishbones or A-arms. A single wheel station, typically representing 1 quarter of a 4-wheel vehicle, has a simplified mechanical representation as shown in FIG. 1, in which the compliance of the tire has been ignored.
In such a traditional passive system, the spring 5 imparts an oscillatory force to the sprung mass 10 (with smooth changes in acceleration and velocity) in response to any motion of wheel 15 (gradual or impulsive). The amplitude of the motion of the mass 10 depends upon the frequency and magnitude of the wheel motion. The effect of the damper 20 is to absorb and dissipate the energy imparted to the system from wheel motions relative to the sprung mass 10.
There are several drawbacks to a traditional passive system. First, the spring-damper system cannot eliminate the transmission of ground irregularities to the sprung mass 10. Second, for much of the frequency range of interest, ground disturbances are magnified by the suspension system, resulting in large spring mass disturbances. Further, choosing spring and damper constants for optimum low frequency ride quality results in poorer high frequency ride quality. Additionally, choosing the best compromise set of constants for vehicle ride quality results in reduced vehicle maneuverability (i.e., poor vehicle handling).
The contradictory choices for passive suspension components lead to the concept of adaptive passive suspension components, usually referred to as semi-active suspension systems. The improvements offered by various semi-active systems have ranged from fair to good. The degree of ride improvement, without simultaneous handling degradation, has been dependent on the responsiveness of the component's adaptability, as well as the control scheme in use. In particular, adaptable dampers, often switching between only 2 states (a high and low damping rate), have achieved some success. In all cases, however, passive components, even if they are adaptive, will still transmit road displacements to the sprung mass. For rough cross-country terrain or road, or for very high performance vehicles, the results are less than satisfactory (Miller and Nobles, 1988; Ivers and Miller, 1989; Miller and Nobles, 1990; Kojima et al., 1991; Pinkos et al., 1993; Hoogterp et al., 1993; Temple and Hoogterp, 1992).
Fully active systems that involve active (bi-directional) force generating components (contrasted with springs, for example, that passively generate restoring forces in response to mechanical displacement), which may be used in conjunction with passive elements, have received far less attention and met with more limited success. This is especially true with respect to suspensions for off-road, rough terrain. The major consideration for on-road, wheeled vehicles is vehicle control over improved surfaces with limited terrain variations. For improved vehicle control over smooth improved surfaces, primary considerations are maintaining constant ground pressure and reducing vehicle roll during turns; passenger acceleration loading is important, but secondary. Consequently, most active suspension control approaches involve measuring forces (such as with a load cell) between the vehicle body and the vehicle suspension attachment points and frequently implicitly assume limited road fluctuations. The major limitation in cross country mobility over rough unimproved, off-road terrain is sprung mass and passenger acceleration loading. Consequently, improving cross-country mobility benefits most from dynamic control of force on the sprung mass, necessitating a somewhat different set of priorities, and assumptions of limited terrain fluctuations are not valid.
Specifically, active suspension success has been limited primarily by the control scheme for the force-generating components, often requiring input of information that is difficult, inconvenient, or costly to obtain. All active and semi-active systems involve a sensing and feedback loop, with various control schemes. Harmonic content of the wheel motion, natural frequencies of the suspension components, speed of calculations, responsiveness of the suspension components, the timeliness of information, and the duration of collecting terrain information before responding, have proven to be critical issues in the quality of results. Typically, for example, assumptions concerning limited road fluctuations are made or information about the terrain in front of the vehicle is needed. Development of such "look-ahead" systems has proven difficult and associated component costs and sizes are prohibitive. Fully active systems without look-ahead capabilities have proven little better (or even worse) than semi-active systems. (Aoyama, 1990; Kiriczi and Kashani, 1990; Dunwoody, 1991; Crolla and Abdel-Hady, 1991; Inagaki et al., 1992; Nagiri et al., 1992; Esmailzadeh and Bateni, 1992).